This invention relates to a recycling process for recovering or reclaiming aluminum metal from aluminum containing impurities such as from aluminum scrap or aluminum skim or dross.
Aluminum metal is an engineering material which can be readily recycled. Aluminum recycling involves the recovery or reclamation of aluminum metal from aluminum scrap containing impurities, e.g., such as aluminum scrap from used beverage cans or aluminum skim or dross from processes wherein molten aluminum metal comes into contact with oxygen in the air. Typically, the impurities in the aluminum scrap or skim exist as oxides and include aluminum oxide on the surface of the aluminum and other oxides such as surface magnesium oxide deriving from alloying elements. Aluminum skim often contains nitrides such as AlN and carbides such as AlC. Other extraneous materials commonly present in used beverage can scrap are silicon dioxide from dirt or titanium dioxide from pigment in the surface coatings on the used beverage cans.
Molten salt reclamation processes involve lifting non-metallic materials from aluminum or aluminum alloys during melting and preferentially wetting the non-metallic materials comprising the impurities in the incoming aluminum scrap. The molten salt preferentially wets the impurities which separate from the aluminum as the aluminum coalesces into metal droplets. The aluminum metal droplets sink to the bottom of a salt-containing process vessel and form into a continuous molten metal pad of aluminum.
The preferential wetting of the oxides and the resulting separation of aluminum from such contaminant oxides is a function of salt composition and the amount of solid particulate oxides and nitrides present in the salt. As scrap is fed to the salt-containing process vessel, the amount of oxides present in the salt increases with each increment of scrap added to the process. This increasing oxide content poses a serious problem in salt-based reclamation processes since the presence of oxides at levels higher than 5 to 30 wt %, as a function of particle size distribution, causes significant reductions in the recovery of aluminum from the process. In effect, the high oxide levels poison the process for reclaiming aluminum metal. This aspect of the conventional processes has been a significant drawback in the efficiency and economics of salt-based recovery systems.
It is an object of the present invention to provide a process for reclaiming aluminum metal from aluminum scrap or skim containing impurities.
It is a further object of the present invention to provide a process for reclaiming aluminum metal from aluminum scrap or skim containing impurities at a higher yield than in present processes.
It is yet another object of the present invention to provide a process for melting aluminum metal while preventing contamination of the melt to an extent that is not now practicable.
These and other objects of the present invention will become apparent from the description of the invention as follows.